This invention generally pertains to multicomponent liquid mixtures including tetraethylorthosilicate, triethylborate, and triethylphosphorus compound.
In the manufacture of semiconductor devices, it is often desirable to dope a silicon layer or structure with boron and phosphorus. Historically, such doping has been performed from separate high purity containers and by co-deposition of boron and phosphorus oxides. More recently, there have been efforts to combine a typical source of silicon, such as tetraethylorthosilicate (TEOS) with trimethylborate and trimethylphosphite to provide a mixture used as a reactant source to directly form borophosphosilicate glass. This mixture had several problems, including transesterification of the components and significant depletion effects, as described by R. A. Levy et al., J. Electrochem. Soc., Volume 134, Number 7, pages 1744-1749 (July, 1987). It was reported that the depletion effects resulted in a fairly rapid decrease in thickness and phosphorus content of the films along the length of the reaction chamber. The authors of this article proposed moving away from TEOS as a reactant, instead employing diacetoxyditertiarybutoxysilane (DADBS) as the reactive component for silicon. However, TEOS continues to be the most commonly used compound for silicate deposition.
Furthermore, other industry trends that promote the use of cocktail mixtures include the growing need for single wafer deposition systems and the use of liquid mass flow controllers (LMFC) for 300 millimeter wafers. LMFCs are used instead of bubblers and simple vapor delivery systems for the transport of the needed dopants to the deposition chamber. Cocktail mixtures are not feasible in bubbler or vapor delivery systems due to the differences in vapor pressures of the individual components of the mixture. The composition of the vapor flow will vary in concentration as the cocktail is consumed and will not result in a repeatable, manufacturable process. Several other issues make bubblers and vapor delivery less desirable and expensive, namely, the poor performance of vapor MFCs due to temperature dependency and condensation issues; temperature controllers for each individual source; expensive "hot boxes" that are required; temperature control of all delivery lines, valves, MFCs, and the like. Similarly, a syringe pump and a controlled leak methodology are not as accurate, have moving parts (particle generator), require frequent maintenance for seal replacement, and have no feedback controls for automation.
On the other hand, LMFCs have been found to be simpler and more controlled. The pure liquid or cocktail mixture is transported to the LMFC where the exact flow is controlled. The use of cocktails in an LMFC is desirable since they are not boiling point dependent, LMFCs do not produce particles during operation, and since LMFCs are considered to be more accurate than syringes. The liquid is then flash vaporized very near to the chamber and delivered via a "transport" gas to the wafer surface. The flash vaporizer is capable of handling multi-component mixtures with no known problems. Accordingly, choice of dopant is not determined by its boiling point, but instead on reactivity and stability. It is expected that use of LMFCs will continue to expand as the film requirements become more strict. The film requirements are a function of the need for thinner films as well as the need to provide repeatability and uniformity for 300 mm wafer processes.
The inventor herein has recognized that a need exists for a multicomponent mixture to serve as a feed stock for borophosphosilicates during semiconductor fabrication. This need is particularly timely given the recent trend toward use of LMFCs during semiconductor fabrication. Such a multicomponent mixture would provide a number of benefits such as simplified delivery systems requiring a single channel for doped silicon oxide production; reduction of process variables due to the simplified system; improved system reliability (i.e., mechanical pumps not being exposed to pure trimethylborate and trimethylphosphite flow during wafer transport), fixed stoichiometry of the reactants which makes the chemical source less dependent on exact calibration of flow controllers, pressures, and efficiency of mixing, and less chemicals to handle.